diff --git a/src/strconv/ftoa.go b/src/strconv/ftoa.go
index eca04b851c..f602d0ffe6 100644
--- a/src/strconv/ftoa.go
+++ b/src/strconv/ftoa.go
@@ -138,6 +138,9 @@ func genericFtoa(dst []byte, val float64, fmt byte, prec, bitSize int) []byte {
 				prec = 1
 			}
 			digits = prec
+		default:
+			// Invalid mode.
+			digits = 1
 		}
 		var buf [24]byte
 		if bitSize == 32 && digits <= 9 {
diff --git a/src/strconv/ftoa_test.go b/src/strconv/ftoa_test.go
index 73008b1c62..3512ccf580 100644
--- a/src/strconv/ftoa_test.go
+++ b/src/strconv/ftoa_test.go
@@ -151,6 +151,11 @@ var ftoatests = []ftoaTest{
 	{498484681984085570, 'f', -1, "498484681984085570"},
 	{-5.8339553793802237e+23, 'g', -1, "-5.8339553793802237e+23"},
 
+	// Issue 52187
+	{123.45, '?', 0, "%?"},
+	{123.45, '?', 1, "%?"},
+	{123.45, '?', -1, "%?"},
+
 	// rounding
 	{2.275555555555555, 'x', -1, "0x1.23456789abcdep+01"},
 	{2.275555555555555, 'x', 0, "0x1p+01"},